1. Field of the Invention
This invention relates generally to a system for manufacturing tire molding metal molds using electrical discharge machining, and more particularly to a system for manufacturing tire mounting metal molds, made of a single material and composed of a single block, with electrical discharge machining using an electrical discharge machining equipment with high accuracy and low manufacturing costs.
2. Description of the Prior Art
In general, tires used for automobiles, etc. have multiple grooves on the tread. Those grooves are usually provided on a tire in such a fashion that they are disposed normal to the curved surface of the tire tread so that they are kept perpendicular to the ground surface when the tire comes in contact with the ground.
In manufacturing molds for molding tires used for automobiles, therefore, it is necessary to form projections which protrude in the direction of the normal lines of the curved surface of the tire tread. This results in complicated mold forming processes. It should be noted that provision of multiple projections on a tire mold causes no problems in releasing the molded tire from the mold since the tire being molded is generally made of resilient rubber.
As described above, tire molds are usually manufactured with complicated processes as follows.
(a) A model of the same dimentions and shape as the radially divided n equal sections of a tire is cast from gypsum, etc. On this gypsum model, patterns or grooves on the tire tread are manually engraved. PA1 (b) Using said gypsum model as a matrix, p (p is smaller than (n) pieces of inversed models are cast from resin, etc. and assembled to form several split models of circular arc shape. PA1 (c) Gypsum is then poured into said split inversed models. The resulting gypsum split models are assembled in a circular shape to form an integral casting mold. PA1 (d) Using said gypsum casting mold as a matrix, a metal mold for molding a tire is cast with aluminum precision casting, etc. PA1 (e) A desired tire metal mold is manufactured by fitting said metal mold into an iron chase having an annular inside surface which engages with the outer circumference of said metal mold.
The tire metal mold manufactured with the aformentioned processes (a) through (e) involves not only high manufacturing costs resulting from complicated manufacturing processes but also the problem of accuracy in assembling p pieces of split inversed molds into a circular shape and the problem of strain in the metal mold caused by the difference of the coefficients of thermal expansion between aluminum of the mold and iron of the chase during heating in molding tires.
To overcome these problems, it has naturally been considered to manufacture a one-piece tire mold made of a single material by means of electrical discharge machining equipment now widely used in machining metal molds. The manufacture of tire molds by electrical discharge machining, however, has the following problems.
In the electrical discharge machining of metal molds such as blanking dies, drawing dies, molding tools, etc. the molding surface of which can be projected from a single direction without shaded-portions, an electrode having a shape corresponding to the molding surface of the metal mold is fed in a predetermined direction and at a predetermined position. However, in manufacturing tire metal molds as shown in FIGS. 1 through 3 with electrical discharge machining, it is impossible to complete metal molds with electrical discharge machining in a single direction. The reasons for this will be described in the following, referring to FIGS. 1 through 3.
In FIGS. 1 through 3, numeral 1 refers to an upper metal mold; 2 to a lower metal mold; 3 to the contour surface thereof corresponding to the tread surface of a tire to be molded; 4 to the shoulder portion thereof; 5 through 9 to projections, respectively. Symbol PL refers to the parting line of the molds along which the upper and lower metal molds 1 and 2 are split; CL to the center line of the tread surface of the tire being molded, respectively.
In general, a plurality of grooves are provided on the curved tire tread surface. Consequently, the projections 6, 8 corresponding to the tire grooves are formed on tire molds, for example on the lower metal mold 2 as shown in FIG. 2 in such a manner that the projections 6, 8 protrude essentially vertical to the curved contour surface 3 which corresponds to the tire tread surface when not in contact with the ground. The machining of the contour surface 3 having more than two rows of projections protruding essentially vertical to the contour surface by feeding an electrode (not shown) having a profile corresponding to the lower metal mold 2 being machined in any direction, for example, in any of the directions shown by arrows a, b and c in FIG. 2 may cause excess metal removal in any one or both of the projections 6 and 8.